To replace the human knee joint, prosthesis types are used whose femoral and tibial parts, depending on the state of preservation of the ligament apparatus, are guided relative to one another with a greater or lesser degree of constraint. This concerns the main degrees of freedom of movement of the knee, namely the flexion movement about a transverse axis, the rotation movement about a rotation axis running approximately parallel to the direction of the tibia, and a translation movement in the anteroposterior direction. The least degree of mutual constrained guidance is to be found in what are called uncoupled prostheses, which are made up simply of a pair of femoral condyles and of a tibial sliding surface. They are used in cases where the ligament apparatus is well preserved. The other extreme is constituted by hinge prostheses, which are used in cases of poorly preserved ligament apparatus and which restrict the possible movements of the knee to the flexion movement (EP-A-42 04 60, DE-OS-29 01 009). Between these extremes there are varying degrees of partially coupled systems comprising, between the femoral part and the tibial part, an intermediate part which, by forming a rotation bearing, takes over the guidance functions for the rotation movement.
Among the partially coupled prostheses which permit a rotation movement, two types are to be distinguished. In the first type, the entire load is transmitted via the intermediate joint component which, in relation to the tibial part, forms a rotation bearing, and, in relation to the femoral part, forms a flexion hinge joint (DE-C-26 60 623). Since in this case the condylar sliding surfaces are intended only for a flexion movement, they can be designed congruent with the opposite surfaces. The opposite surfaces are therefore made concave with the same radius of curvature. The second type of partially coupled prostheses transmits the load not via the intermediate joint component, but directly from the condylar sliding surfaces to tibial sliding surfaces cooperating with these (EP-A-174 531). In this case, not only does a flexion movement take place between the condylar sliding surfaces and the tibial sliding surfaces, but also the rotation movement. For this reason, the tibial sliding surfaces should not be made congruent with the condylar sliding surfaces. If they are to permit a free rotation movement, the tibial sliding surfaces have to be flat. In general, however, they are allowed to slope slightly upward in front of the area in which they cooperate with the condylar surfaces when the femoral part and the tibial part have the same anteroposterior alignment (area of normal contact). This has the effect that, in the event of rotation, the condylar sliding surface displaced forward in relation to the tibial sliding surface during the rotation is lifted. This generates, under the load transmitted from the joint, a restoring torque which ensures that the prosthesis parts, as soon as is possible, return to their normal position of having the same anteroposterior alignment. During the rotation relative to the tibial part and the thereby obtained lifting of the femoral part, the rearwardly migrating condylar surface loses its contact with the tibial sliding surface. The entire load then has to be transmitted on the other side, which leads not just to increased wear, but also to an undesired bending moment in the area of the rotation bearing. It is from this prior art that the invention starts out.
In a known publication (DE-A-41 02 509), a partially coupled prosthesis is discussed in which both the flexion movement and also the rotation movement takes place between the femoral and tibial sliding surfaces. The femoral sliding surfaces are rounded convexly in the sagittal and frontal plane. From the view of the tibial sliding surfaces, it can be concluded that they are identical to and complement the shape of the femoral sliding surfaces. This permits a rotation movement of the knee components relative to one another about the flexion axis, but provides high resistance to a rotation moment about the axis parallel to the direction of the tibia. The known prosthesis is therefore not suitable for such a rotation movement. Were such a rotation to take place, however, the femoral sliding surfaces would spring out of the tibial slide depressions and there would no longer be any stability of the prosthesis against further rotation. In addition, the femoral sliding surfaces would lie only on the edges of the tibial slide depressions and deform these under load. The publication provides no information on how the sliding surfaces have to be configured so that they ensure both the possibility of rotation and also stability upon rotation and so that, in the event of rotation, force can be transmitted in a way that does not damage the prosthesis.